1. Field of the Invention
This invention relates in general to halftone image processing, and more particularly to a halftone method and apparatus that provides simultaneous, multiple lines per inch screens.
2. Description of Related Art
As an approximation to continuous tone images, pictorial imagery is represented via a halftone image processing apparatus and process in which each input picture element (pel) is translated into a j×k pattern of recorded elements, where j and k are positive integers. Herein, it will be understood that separate threshold matrices are used for each color component. However, as will become obvious from the discussion that follows, the color components will not be considered together. Rather, the term “pel” will be used to refer to “samples” in an array. A halftone image is reproduced by printing the respective pels or leaving them blank. That is, by suitably distributing the recorded elements.
Image processing apparatus and processes are evaluated in part, by their capability of delivering a complete gray scale at normal viewing distances. The capability of a particular process to reproduce high frequency renditions (fine detail) with high contrast modulation makes that procedure superior to one which reproduces such fine detail with lesser or no output contrast.
Another measure of image processing apparatus and process merit is the tendency to produce visual artifacts in the output image that are not part of the original image, but are the result of the image processing, including moiré patterns, false contours, and false textures. Moiré patterns are false details created most often by the beating between two relatively high frequency processes resulting in a signal whose spacial frequency is low enough to be seen by the viewer. False contours are the result of gray scale quantization steps which are sufficiently large to create a visible contour when the input image is truly a smooth, gradual variation from one to the other. False textures are artificial changes in the image texture which occur when input gray levels vary slowly and smoothly and the output generates an artificial boundary between the textural patterns for one gray level and the textural patterns for the next gray level.
Halftone threshold matrices are used to convert grayscale images and graphics into binary images. The density of the picture elements (pels) in a region is indicative of the original values in that region. Originally, these matrices contained one basic cell in which at most one additional pel was turned on with an increment to the next input value. Larger basic cells allowed more shades to be represented, but cut down on the amount of detail that could be preserved since they had a coarse screen frequency (i.e., generated patterns with a low number of lines per inch in the halftone screen). A problem exists with the number of density levels attainable with a limited resolution and acceptable screen frequency. One way to get more gray levels is to reduce the number of lines per inch and adoption of larger matrix dimensions, but this decreases the resolution and decreases the screen frequency.
Later, multiple basic cells were grouped together into a supercell. The supercell approach attempts to improve both gradation and resolution. This method is generally referred to as the “Improved Halftone” (IH) method.
For example, in the IH method, an 8×8 superthreshold value matrix may be divided into four 4×4 submatrices. The small matrix is adopted for the resolution unit, and the large matrix is adopted for the gradation unit. Thus, the amount of detail is determined by the basic cell size while the number of shades depends upon the supercell size. As the supercells have become larger, more than one pel inside the supercell is turned on for the next input level. However, never more than one pel is turned on at a time inside a basic cell for the next constant input level. The patterns generated by repetition of the original basic cell are contained in the patterns generated by the supercell.
It can be seen that there is a need for a halftone method and apparatus that provides simultaneous, multiple lines per inch screens.